Methods for fabricating highly integrated semiconductor devices are disclosed and, more particularly, methods for forming polyatomic layers of a semiconductor device are disclosed.
Generally, as semiconductor devices become highly integrated and miniaturized, the area occupied by the constitutional elements gets smaller. Although as the size of a semiconductor device shrinks, however, a minimum capacitance should be secured to drive the device.
When fabricating a capacitor of a 64 Mbyte or 256 Mbyte-DRAM using a conventional dielectric material such as SiO2 or Si3N4, the area occupied by the capacitor should be more than six times larger than the cell area to secure the essential capacitance, even though the SiO2 or Si3N4 layer is made as thin as possible. As a planar capacitor cannot fulfill this condition, a method for increasing the charge storage area is sought.
Many structures including a stack capacitor, a trench capacitor, or a hemispheric polysilicon layer have been suggested to increase the charge storage area or, in other words, to increase the storage node surface area of a capacitor. However, in case the structure of a capacitor is made complicated just to increase its charge storage area, there are problems that the production cost goes up and that the efficiency declines due to the complex manufacturing process.
Therefore, it is hard to apply a method of increasing the charge storage area of a capacitor by forming it in three-dimensional and fulfilling capacitance to a DRAM device over a 1 Gb class.
To solve these problems, studies have been conducted on the Ta2O5 dielectric layer so as to substitute the conventional SiO2/Si3N4 dielectric layer, but the capacitance of the Ta2O5 layer is no more than two to three times that of the SiO2/Si3N4 dielectric layer. Accordingly, to employ a Ta2O5 dielectric layer to a highly integrated DRAM, the thickness of the dielectric layer must be reduced. But, this Ta2O5 dielectric layer creates a problem as the amount of leakage current increases.
For this reason, a high dielectric thin film is needed to fabricate a capacitor for 1 Gb DRAMs. When using a thin film with a high dielectric constant, it""s possible to obtain adequate capacitance only by a planar capacitor, thus simplifying the manufacturing process.
A(Ba,Sr)TiO3 (hereinafter referred as BST) layer has been studied a lot as a high dielectric material. The capacitor adopting the BST layer has a capacitance dozens of times as big as that of adopting the SiO2/Si3N4 group as well as the structure and thermal stability of the capacitor adopting SrTiO3 and the excellent electric property of the capacitor adopting BaTiO3, which makes it an appropriate material for a memory device of over a 1 Gb class.
Among other materials having high dielectric constants, the BST layer of a perovskite structure is appropriately applicable to a high-density and high-integrated capacitor, which requires a high dielectric constant and small leakage current. This is because the BST layer features a high dielectric constant and superb insulation property with low dielectric dispersion and dielectric loss at a high frequency, and existing in a paraelectric at a room temperature. Furthermore, the BST layer doesn""t have the problem of fatigue or degradation.
A polyatomic layer is formed with a sputtering deposition method, a chemical vapor deposition(CVD) method or an atomic layer deposition(ALD) method.
For forming a layer with the sputtering method, a high voltage is supplied to a target and inactive gases are injected to a vacuum chamber in order to generate plasma. For example, if an Ar gas is injected as the inactive gas, Ar ions are generated. The Ar ions are sputtered to the surface of the target and atoms are parted from the surface of the target. By this sputtering method, a thin film having high purity and good adhesion to a substrate may be deposited. However, in case of forming the multitudinous thin film composed of various atoms with the sputtering method, it is not easy to obtain the uniformity of the thin film, because the various atoms need the different optimum condition for depositing. Therefore, the sputtering method has limitations to be applied to form fine patterns.
The CVD is most widely used deposition method, a reaction gas and a source gas are used to form a thin film on a substrate to a required thickness. That is, various gases are injected to a reaction chamber, the gases excited by heat, light or plasma, chemically react each other, and the thin film is formed. In the CVD method, the deposition rate may be increased by controlling the deposition conditions, such as the plasma, chamber temperature and the ratio of reaction and source gases. However, it is difficult to control thermal stability of the atoms because of the rapid gas reaction and the physical and chemical properties of the thin film are deteriorated.
In the ALD method, a reaction gas and a purge gas are supplied alternately to form an atomic layer. The atomic layer formed with the ALD method shows good step coverage even if the atomic layer is formed on a structure having high aspect ratio, and it is possible to obtain a uniform layer at low pressure condition and to improve the electrical characteristic of the layer.
Lately, as the integration of the semiconductor device increases, the capacitor is formed with structures such as cylinder, fin, and stack structure, or is formed with a hemi spherical polysilicon layer in order to store much more charges in a small area. That is, the structure of charge storage electrodes of capacitors become complicated, therefore dielectric layers is formed with deposition methods, such as the ALD method, capable of guaranteeing good step coverage.
The ALD method use chemical reactions like as the CVD method, however the ALD method is distinguished from the CVD method in that the reaction gases are injected to a reaction chamber one by one without mixing between the reaction gases. For example, a gas A and a gas B are used as the reaction gases, firstly, the A gas is injected, and molecules of injected gas A are absorbed chemically on a substrate. Thereafter, inactive gases, such as Ar and N2 are injected to the reaction chamber in order to purge the gas A remaining in the reaction chamber, and the gas B is injected to the reaction chamber. The injected gas B reacts with the molecules of the gas A only on the substrate, and an atomic layer is formed on the substrate. And then, the remaining gas B and accessory products are purged. The thickness of a layer is controlled by the repetition of the above-mentioned processes. Namely, the thickness of a layer formed by the ALD method closely relates to the number of repetition time.
Generally, the BST layer or STO layer is formed by the CVD method among above mentioned various deposition methods, and it is known that the BST layer has a best dielectric characteristics when the atomic ratio of Ba:Sr:Ti in the BST layer equals to 25:25:50. Therefore, in case of using the CVD method for forming the BST layer, for the purpose of obtaining intrinsic dielectric and excellent leakage current characteristics, it is needed to develop precursors and optimize the deposition condition in order to get the atomic ratio of Ba+Sr:Ti in the BST layer equal to 1:1. Hereinafter, Ba+Sr denotes a sum of atomic ratios of Ba and Sr.
FIG. 1 shows atomic ratio Sr/Ti dependency on step coverage, when a STO layer is formed in a three dimensional contact whose critical dimension CD is 0.15 xcexcm. In FIG. 1, atomic ratio Sr/Ti dependencies of two STO layers are shown, that is, one STO layer is formed by flowing Sr source at a rate of 0.03 ml/min and Ti source at a rate of 0.1 ml/min (denoted to Sr:Ti=0.03:0.1 ml/min), and the other STO layer is formed by flowing Sr source at a rate of 0.045 ml/min and Ti source at a rate of 0.15 ml/min (denoted to Sr:Ti=0.034:0.15 ml/min). The two STO layers are formed with Sr(THD)2-pmdt as a Sr precursor and Ti(THD)2(O-i-Pr)2 as a Ti precursor. Generally, it is known that the STO layer has a best dielectric characteristics when the atomic ratio of Sr:Ti in the STO layer equals to 1:1, however, as shown in FIG. 1, the atomic ratios are varied extremely in accordance with the height.
THD and pmdt represent tetramethylheptanedionate and pentamethyl-diethylenetriamine, respectively. Also, O-i-Pr stands for isopropoxide. Tetraen or tetraene mean a structure with four double bonds; trien or triene mean a structure with three double bonds. Further, THD, MPD and pmdt are represented to chemical equations of C11H19O2, O2C6H12 and C9H23N3, respectively.
FIG. 2 shows atomic ratio Ba+Sr/Ti dependency of step coverage, when a BST layer is formed in a three dimensional contact whose critical dimension CD is 0.15 xcexcm. The BST layer are formed with Ba(METHD)2 as a Sa precursor, Sr (METHD)2 as a Sr precursor and Ti (MPD) (THD)2 as a Ti precursor. As mentioned above, the BST layer has a best dielectric characteristics when the atomic ratio of Ba+Sr:Ti in the BST layer equals to 1:1, however, as shown in FIG. 1B, the atomic ratios are varied extremely in accordance with the height.
METHD and MPD represent methoxyethoxyte-tetramethylhepatanedionate and methylpentanediol, respectively.
A BST layer or a STO layer may be formed on a lower electrode as a dielectric layer by the CVD method in the processes for forming a three-dimensional capacitor, the atomic ratio of Ba+Sr:Ti or Sr:Ti is varies in accordance with a deposition height because of the deposition differences of Ba, Sr and Ti elements, moreover as the decrease of the design rule, the variation of the atomic ratio becomes larger. If the atomic ratios of the BST layer or the STO layer are quite different from each stoichiometry, the BST layer or the STO layer cannot have the perovskite structure even after an annealing process, and it is impossible to increase the dielectric constant of each layer.
Therefore, it is worthy to note that the ALD method capable of overcoming the problems of composition variation by height change. FIG. 3 shows steps of forming polyatomic layer using the conventional ALD method.
Referring to FIG. 3, a first precursor containing parts of source elements, for example containing Ba and Sr for forming a BST layer, is flowed into a reaction chamber and is absorbed on a substrate on which a polyatomic layer to be formed. Thereafter, remaining first precursors are purged out, and a first reaction gas is flowed into the reaction chamber in order to induce a surface reaction with the first precursor absorbed on the substrate, thereby forming a fist unit layer. And then, remaining first reaction gas and accessory products are purged out.
Subsequently, a second precursor containing other parts of elements, for example Ti for forming a BST layer, is flowed into the reaction chamber and, is absorbed on the substrate on which the first unit layer is already formed. Thereafter, remaining second precursors are purged out, and a second reaction gas is flowed into the reaction chamber in order to induce a surface reaction with the second precursor absorbed on the substrate thereby forming a second unit layer. And then, remaining second reaction gas and accessory products are purged out.
As mentioned above, in order to form a polyatomic layer, such as BST or STO, a common process condition in which each element can be deposited is needed. However, it is difficult to obtain a common process condition in the ALD method, because characteristics, such as volatility, thermal decomposition, deposition speed, incorporation efficiency and sticking coefficient, differ greatly with each of the metal organic precursors.
In case of forming a BST layer with an ALD method, a xcex2-diketonate group material, which can be used as a precursor of Ba and Sr, does not react with O2 or H2O, at a low temperature less than 350xc2x0 C., therefore it is impossible to obtain a atomic deposition processes at the temperature with xcex2-diketonate. On the contrary, it is possible to obtain atomic deposition with Ti (OC3H7)4 used as a precursor of Ti at a temperature of 150xc2x0 C. to 300xc2x0 C. However Ti(OC3H7)4 is re-decomposed around temperature of 300xc2x0 C., therefore, it is impossible to obtain atomic deposition at such a temperature with Ti(OC3H7)4.
As mentioned above it is necessary to make the atomic ratio of Ba+Sr:Ti nearly equal to 1:1 in the BST layer for securing the best dielectric characteristic of BST. Therefore, one atomic unit layer, formed of a precursor containing Ba and Sr, and the other atomic layer, formed of a precursor containing Ti are repeatedly deposited in order to make the atomic ratio of Ba+Sr:Ti equal to 1:1.
However, due to the respective characteristic of the precursors, the deposition temperatures are should be changed. That is, in the step of using the precursor containing Ba and Sr, the reaction chamber should be maintained to a temperature of about 350xc2x0 C., and in the step of using the precursor containing Ti, the reaction chamber should be maintained to a temperature of 150xc2x0 C. to 300xc2x0 C. Actually, it is impossible to change of the reaction chamber in every step for forming each atomic unit layer. Whatever it possible to change the temperature of the reaction chamber in every step, the problems of increasing the number of process step and lowering deposition speed exist.
Therefore, a method for forming a polyatomic layer, capable of reducing a number of process steps and increasing deposition speed is disclosed.
A method for forming a polyatomic layer with a mixed deposition method consisting of an atomic layer deposition method(ALD) and a chemical vapor deposition method is also disclosed.
In accordance with a disclosed embodiment, a method for forming polyatomic layer comprises: forming a first unit layer having a first element of the polyatomic layer using an atomic layer deposition; and forming a second unit layer having a second element of the polyatomic layer using an chemical vapor deposition.
In accordance with another embodiment, a method for forming polyatomic layer comprises: performing a process for a first precursor being absorbed on a substrate in a chamber, wherein the first precursor containing a first element of the polyatomic layer; purging out the first precursor in the chamber; supplying a first reaction gas in the chamber and forming a first unit layer by inducing a surface reaction between the first reaction gas and the first precursor on the substrate; purging out the first reaction gas and a accessory product in the chamber; supplying a second precursor and a second reaction gas in the chamber and forming a second unit layer, wherein the second precursor containing a second element of the polyatomic layer, and wherein the second precursor and the second reaction gas reacts chemically; and purging out the second precursor, the second reaction gas and a accessory product in the chamber.
Mixed deposition methods consisting of an atomic layer deposition(ALD) method and a chemical vapor deposition(CVD) method for forming a polyatomic layer are also disclosed.
Some elements are deposited by the ALD method, and others are deposited by the CVD method for forming a polyatomic layer. Among various elements, some elements having a higher deposition temperature compared with others are deposited with the ALD method, and the others are deposited the CVD.
In this mixed deposition method, various elements having respective characteristics, such as thermal decomposition, deposition speed, incorporation efficiency and sticking coefficient, are deposited independently with the ALD method and the CVD method. Therefore, it is possible to obtain characteristics, such as good step coverage and abilities of containing impurities and controlling composition, provided by the ALD. In addition, it is possible to improve the deposition speed and to shorten the process time by the CVD.